Use tcpdump or ethereal to capture some real HTTP requests. Save them to a file and replay the requests through Squid. Feel free to modify or delete some of the HTTP headers.
Try to make Squid run out of file descriptors.
Run tail -f cache.log and start Squid with debug_options set to ALL,3. If that is too overwhelming, try ALL,2.
Force Squid to generate a core file by sending each of the following signals: SIGBUS, SIGSEGV, and SIGABRT. Find the core file and use gdb or another debugger to get a stack trace.